A carbon nanotube is a tubular carbon polyhedron having a structure where a graphite sheet forms a cylindrical shape. Among carbon nanotubes, there are multi-layer nanotubes having a multi-layer structure, where a graphite sheet forms a cylindrical shape, and single-layer nanotubes having a single layer structure where a graphite sheet forms a cylindrical shape.
The multi-layer nanotube was one of the nanotubes discovered by Iijima in 1991. Iijima discovered that multi-layer nanotubes were present in a carbon mass deposited on a cathode in an arc discharge process (S, Iijima, Nature, 354, 56 (1991)). Thereafter, research on multi-layer nanotubes was conducted. In recent years, the state of the art has reached such a level that multi-layer nanotubes can be synthesized in large quantity.
On the other hand, single-layer nanotubes have an inner diameter of about 0.4 to 100 nanometers (nm). Reports on the synthesis of single-layer nanotubes were simultaneously issued by Iijima and a group at IBM in 1993. The electronic state of a single-layer nanotube is predicted theoretically, and it is considered that the electronic physical properties vary from metallic to semiconductive depending on how the helix is wound. Accordingly, single-layer nanotubes are promising as a future electronic material.
As to other applications of single-layer nanotubes, nanoelectronics material, a field electron emitting emitter, a high directionality radiation source, a soft X-ray source, one-dimensional conductive material, high heat conductive material, hydrogen occluding material, and the like, are considered. Further, it is thought that the application of single-layer nanotubes can be further expanded by including functional groups on the surface, metal coating thereof, and including foreign material.
Conventionally, single-layer nanotubes are produced by mixing metals such as iron, cobalt, nickel or lantanum into a carbon rod anode and performing arc discharge (Patent Literature 1).
In the production method using arc discharge, in addition to single-layer nanotubes, multi-layer nanotubes, graphite, and amorphous carbon are mixed in the final product, and the yield of the single layer nanotubes is not only low, but also fluctuation occurs in yarn diameter and yarn length. Therefore, it is difficult to produce single-layer nanotubes with relatively equal yarn diameters and yarn lengths.
Besides the arc discharge process, for the production of carbon nanotubes, a vapor phase cracking, a laser sublimation process, an electrolytic process of a condensed phase and the like have been proposed (see for example, JP-A-06-280116, Japanese Patent No. 3100962, JP-A-2001-520615, and JP-A-2001-139317).
However, the respective production methods disclosed in these publications are directed to methods performed in a laboratory or on a small-scale level, and they include the problem that the yield of carbon material is low.
Further, in the methods described above, it is difficult to implement a stable mass production such that production can not be performed continuously.
On the other hand, the recent availability of carbon material with nano-units (so-called carbon nanofiber) is expected, and it is desired that the carbon material can be mass-produced industrially.
In view of the problems of the prior art, the problem to be solved by the present invention is to provide a production method and apparatus for carbon nanofibers, which allows efficient production of carbon material with nano-units by a simple process.